Skype - With the Skype app, you can make free Skype-to-Skype calls, and send and receive IMs for free, one-to-one or with a group. You can also call phones at Skype’s great rates – over WiFi or 3G.
To make calls to phones abroad, simply dial the number in Skype and pay with Skype Credit – or make unlimited calls with a subscription...

MobileVOIP - With MobileVoip you can make free* voip calls from your mobile phone. Make calls to any country you desire at the cheapest rates. MobileVoip is currently supported on iPhones, Android phones and Symbian phones. Simply download the free MobileVoip app from one uour APP market and register with one of the supported Voip brands...

Start a group, add your friends, and we’ll give you a free phone number for it. GroupMe is the easiest way to start conversations and keep in touch with your real life network. It's totally free, and it works on every phone...

3CXPhone (Android) is a free VoIP Softphone that works with SIP based IP PBXs such as 3CX or Asterisk and popular VoIP providers. 3CXPhone is not locked to a particular provider so you can switch easily.
What's New in This Release:
· includes support for tablets and has more intuitive in/out of office icon
What's New in 1.3...